Autosomal DNA

Autosomal DNA is a term used in genetic genealogy to describe DNA which is inherited from the autosomal chromosomes. An autosome is any of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X chromosome and the Y chromosome). Autosomes are numbered roughly in relation to their sizes. That is, Chromosome 1 has approximately 2,800 genes, while chromosome 22 has approximately 750 genes. There is no established abbreviation for autosomal DNA: atDNA (more common) and auDNA are used.

The AncestryDNA test is only available in the United States, the United Kingdom and Ireland. The 23andMe test is available in 56 countries. The Family Finder test is available worldwide. For further information see the article on shipping DNA kits, which includes information on shipping costs together with a list of package forwarders. Note that if you are in the US and live in New York or Maryland you will not be able to order a 23andMe test, though you will be able to purchase the AncestryDNA test or the Family Finder test.[1][2]

Who to test?

For autosomal DNA testing one should always test the oldest generations first wherever possible - your parents, grandparents (if you are lucky), aunts and uncles. By testing yourself as well as your parents you will be able to determine which segments have been inherited from which parent, and you will also be able to rule out coincidental (Identical by state) matches. A two-parent/child trio also provides the best results for the purposes of phasing and chromosome mapping.

If you only have one parent available for testing then you should test you and your parent. You should also test your siblings because they will inherit part of your parents' DNA that you don't carry.

Your uncles and aunts will inherit part of your grandparents' DNA that your parents don't have.

The next priority should be to test other close relatives from first to third cousins to get the best representation across your ancestry. Experience suggests that the maximum return is obtained by testing second cousins. They share one set of great grandparents so when someone matches you and a second cousin you get to narrow down your matches to that specific line.

For the purposes of chromosome mapping you would need to test four 2nd cousins in order to get the same amount of DNA mapped as would be the case if you just tested one first cousin. However, the segments mapped with second cousin data would be attributed back two generations whereas the segments mapped with first cousin data can only be attributed back one generation.

Introduction to Autosomal DNA

Maurice Gleeson provides an introduction to autosomal DNA testing and the matching process in the following video:

Other resources

DNA Adoption The DNA Adoption website has many useful resources. Although the website was primarily set up to help adoptees get the most out of their autosomal DNA results, the methodology described is equally applicable for genealogists.